The present invention relates to a resonator arrangement comprising at least two folding elements for folding the beam path, the folding elements serving to restrict the divergence angle of the radiation.
The simplest optical resonator is the so-called Fabry resonator which consists of mirrors arranged in parallel with one another. The beam that is incident on the mirror in vertical direction is reflected back and forth without any directional change between the two mirrors that are arranged in plane-parallel fashion. When a gain or amplification medium is placed between the two resonator mirrors, the beam can be highly amplified. It is coupled out through one of the two mirrors that is made partially transmissive for this purpose. Other rays impinging on the resonator mirrors in a direction other than the one of vertical incidence are reflected out of the resonator after a few passes or travels. As a result, the beam vertically impinging on the resonator mirrors experiences a maximum gain. As a consequence, this beam prevails over the other rays. The resonator thereby selects the beam perpendicular to the mirrors because of the higher gain.
In practice, there are many different types of lasers in which media are used with different characteristics for amplification or gain. For instance, there are laser media with a low gain factor, such as Cr:LiSaF, and laser media with an extremely high gain factor, as e.g. in the case of the diode laser. For laser media with a low gain factor, high-quality resonators must be constructed to ensure an efficient laser operation. In such a case a resonator which consists of two mirrors is often adequate for achieving the necessary mode selection.
The diode laser, on the other hand, has an extremely high gain factor. To achieve a high efficiency, a large part of the radiation traveling in the resonator must be coupled out. For instance, the resonator of a semiconductor laser typically consists of an end mirror having a reflection of about 95% and an output coupler with a reflection of about 5%. In addition, there is a high gain in comparison with conventional lasers. As a consequence, the resonator quality is minimum and the mode selecting capacity is therefore low. This means that modes of a low beam quality are also created.
An enhanced mode selection can e.g. be achieved by way of an aperture. There are in principle two different types of apertures: mode apertures on the one hand and soft gain apertures (gain guiding) on the other hand.
For an enhanced mode selection in diode lasers, use is made in U.S. Pat. No. 5,231,642 of periodic grating structures or distributed reflection structures. The structure used can be produced by way of a photolithographic process. The function of the periodic grating structures or of the distributed reflection structures consists in enhancing the mode selecting capacity on the one hand and in reducing filamentation on the other hand. The reflection on the grating structures, as are known from U.S. Pat. No. 5,231,642, is based on constructional interference according to the Bragg principle. As a rule, such gratings or grating structures are difficult to manufacture; there are great losses on the boundary surfaces, in particular in the case of an inaccurate manufacture; these may result in relatively large subassemblies, and finally gratings show a wavelength dependence on the reflection characteristics.
Starting from the above-described prior art, it is the object of the present invention to indicate a resonator arrangement by which the divergence angle of the radiation can be changed or reduced within the resonator arrangement independently of the wavelength.
This object is achieved in a resonator arrangement of the above-mentioned type in that the respective folding is caused by reflection on a reflecting surface of the respective folding element, the beam axis of the radiation and the surface normal of the respective reflecting surface being positioned at an angle relative to each other that is greater than the critical angle for the total reflection but smaller than the sum of the critical angle for the total reflection plus the divergence angle of the radiation.
Such an arrangement is characterized in that a strongly angle-dependent reflection degree is created on the whole so that the divergence angle of the radiation traveling in the resonator of such a design is limited without the need for resorting to the Bragg reflection on a grating having the above-mentioned drawbacks.
In a preferred development of the resonator arrangement, the respective reflecting surface is formed by the boundary surface between materials having different refractive indices. This arrangement is particularly suited for solid-state lasers having a glass-like or crystalline gain medium and for semiconductor lasers.
Furthermore, it is of advantage that in the resonator arrangement the waves are guided in part by a waveguide structure and that the respective reflecting surface is formed by changing the profile of the waveguide structure. Such an arrangement should always be used when e.g. the height of the waveguide profile can be varied in a simple way. For example, the waves are guided in semiconductor lasers in vertical direction by a layer structure which is designed as a waveguide. The waveguide profile can be changed in this instance by etching away upper layers.
To achieve a compact structure and a radiation which is only coupled out in one directionxe2x80x94also without a reflecting coating of a resonator end surface, at least the folding elements are arranged with respect to one another such that a resonator end mirror and a resonator output coupler are positioned in one plane.
In a further preferred embodiment, at least one of the folding elements of the resonator arrangement are provided with a plane-parallel layer which is partially transparent for the radiation. This partially transparent plane-parallel layer should here be constructed such that part of the impinging radiation is guided in this layer in the manner of a waveguide and the radiation is thus transported in a direction transverse to the preferred propagation direction of the radiation. This can contribute to a homogenization of the radiation field.
Preferably, the gain region is designed such that it only occupies part of the resonator volume formed by simple or multiple folding.
To avoid the formation of undesired radiation distributions, the regions which are positioned within the resonator volume and are not designed as a gain zone are provided with an additional absorption.
In a preferred variant of the resonator arrangement, a gain zone is provided which is layered, folding being performed in a plane perpendicular to the layer. Such a resonator arrangement entails advantages to the effect that despite the small thickness of the gain region the radiation can be coupled out over a comparatively much larger cross-sectional area.
There may be cases where the demand is made that the extension of the gain regions should remain below given limit values to permit e.g. an improved heat discharge or dissipation. To comply with such a demand, a separate gain region is respectively arranged in the area of individual folding elements.
To achieve an additional beam shaping, in particular for influencing the divergence angle, individual surfaces of the reflecting surfaces may be provided with an additional curvature.
For the construction of a ring resonator at least three suitably positioned folding elements that are aligned with one another may be provided.
Furthermore, a gain medium may be arranged in parallel with the reflecting surfaces of at least part of the folding elements. This has also the advantage that the beam filamentation can be reduced in the gain regions.
In combination with a resonator arrangement in which at least one of the folding elements is provided with a plane-parallel layer which is partially transparent for the radiation, this plane-parallel layer can be formed by a further boundary surface between media having different refractive indices.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.